Parkinson's disease (PD) affects 1% of the population over age 65. Psychotic symptoms occur in up to 40% of patients, and the majority of PD patients with psychosis also have dementia (up to 80%). Psychosis in PD is exceptionally challenging to manage since the development of psychotic symptoms will constrict the dose of antiparkinsonian medications, dose reductions or withdrawal of antiparkinsonian medications exacerbate motor symptoms, and antipsychotic medications can aggravate parkinsonism and worsen cognitive function. There is a crucial need for empirical data concerning safe and effective treatments of psychosis in PD. "Atypical" neuroleptic (antipsychotic) agents are potentially useful agents for treating psychosis in PD since their pharmacological profile renders them less likely to cause extrapyramidal symptoms than conventional antipsychotic medications, e.g., haloperidol or chlorpromazine. Olanzapine, released for treatment of psychosis in October, 1996, has potential applications in PD, but its dopamine blocking properties may increase the risk of adverse motor effects. This is clearly the case in some patients with PD, but it is unclear whether the presence or degree of dementia changes the tolerability to olanzapine. Accordingly, the overall aim of this investigation is to conduct a 6-week trial of olanzapine starting at 2.5 mg qhs to examine the efficacy, safety, and cognitive effects of olanzapine for treatment of psychosis in patients with PD and moderate to severe dementia The projected sample size is 15 subjects.